One of the factors that limit the capacity of a cellular network is the amount of interference, both inter-cell and intra-cell, that a wireless communications network can tolerate. There are two ways to keep interference below a tolerable level. One is to have a frequency re-use plan that maintains a minimum distance between co-channel cells, and the other is to underutilize the wireless spectrum. However, both methods reduce the usage efficiency of the radio spectrum.
Because the demand for wireless communications has grown dramatically over the years, the radio spectrum has become crowded. It is absolutely essential for a wireless communications network to deploy some type of interference mitigation or suppression method to increase the reuse rate of a radio frequency so that the available radio frequencies can be fully utilized.
A base transceiver station (BTS) equipped with an antenna array system provides an effective means of mitigating or suppressing interference. By applying a beamforming method to an antenna array, the antenna beam pattern is positioned toward the desired signal. In an uplink direction, the beamformed receiving antenna beam pattern points to the desired signal while the null of the antenna beam pattern is positioned towards the interference. In the downlink direction, the beamformed transmitting antenna beam pattern points to the intended receiver while the null of the antenna beam pattern is positioned towards other users. By employing the null-steering beamforming method on both the uplink and downlink directions, the interference in the wireless communications network is effectively suppressed.
In a null-steering beamforming method, the calculation of a beamforming weighting vector takes into account the spatial signatures of both the desired and interference signals. For example, the beamforming weighting vector is obtained by solving the following eigenvalue problem: (Ri+σn2J)−1Rs·w=λw, where Ri is the covariance matrix of interference; σ is the standard deviation of background noise; Rs is the covariance matrix of the desired signal; and I is the identity matrix. The beamforming weighting vector is the primary eigenvector corresponding to the largest eigenvalue of the eigenvalue problem.
The accuracy of the spatial signatures of the desired signal and the interference signal is an essential factor for finding an optimal beamforming weighting vector in a null-steering beamforming method. The spatial signature of the desired signal of customer premises equipment (CPE) can be obtained from data traffic and designated uplink sounding signals between the BTS and the CPE.
Uplink sounding is a mechanism designed to facilitate the determination of the spatial signature of a communication channel between the BTS and the CPE. The BTS selects a special region in an uplink frame for a CPE to transmit sounding signals. The BTS calculates the spatial signature of the CPE using the detected sounding signals. The spatial signature is subsequently used to calculate the beamforming weighting vector of the CPE.
A sounding signal is transmitted at a frequency that is as close to the frequency for sending data traffic as possible, if not the same, in order to minimize the loss of accuracy due to the frequency mismatch. In one form, the BTS selects a region in the beginning of the uplink frame for sending uplink sounding signals. The BTS instructs the CPE to send uplink sounding signals within the designated sounding region.
Similarly, the BTS allocates a region at the end of the uplink frame for sending downlink sounding signals. The BTS instructs the CPE to send downlink sounding signals within the specified sounding region. The uplink and downlink sounding regions can be merged into one. The sounding signals transmitted in the combined region are used to calculate both the uplink and downlink beamforming weighting vectors of the CPE.
The spatial signature of a communications channel between the BTS and the CPE can be calculated from the sounding signals sent in the designated sounding region. However, there is no easy way to obtain the spatial signature of the interference because it is rather difficult to separate the interference from background noise. In addition, the interference from neighboring cells is not synchronized in time or frequency with the desired signals.
As such, what is desired is a method and system for detecting desired signals as well as the interference signals, thereby achieving effective null-steering beamforming.